This invention relates to sensors and to control systems incorporating sensors.
A sensor generates measurement data. Typically, the measurement data is a signal that estimates the value of a process variable. In practice, the signal does not perfectly represent the value of the process variable. Instead, the signal also includes effects resulting from the sensor (such as sensor faults or distortion) and other process influences (including those attributable to "faulty" process behavior).
Sensor and control system designers employ numerous techniques to increase the reliability of measurement data. For example, sensor designers try to develop improved sensor designs that minimize both the occurrence of sensor faults and the distortions occurring during normal operation. In another approach, control system designers implement rigorous programs of sensor checking, maintenance, and calibration to reduce both the frequency at which sensor faults occur and the distortion caused by poorly maintained sensors.
In conjunction with increasing the reliability of measurement data, designers employ fault detection techniques to increase a control system's ability to recognize that measurement data is unreliable. For example, control system designers often rely on sensor redundancy to reduce the effect of any sensor fault that may occur. If measurement data from a sensor in a group of redundant sensors is inconsistent with measurement data from other sensors in the group, a control system can designate the inconsistent data as unreliable and ignore that data.
In another approach to fault detection, control systems monitor information about the process and the sensor for signs of sensor faults. Until recently, sensors have been limited to a single analog communication channel, normally based on the 4-20 mA convention, and, therefore, have been unable to transmit signals other than a measurement data signal. Faced with this limitation, users of process fault detection techniques have tried to extract sensor and process fault information from measurement data. However, in attempting to minimize distortion of the measurement data, sensor designers have tried to eliminate, by sophisticated filtering and other means, every component of the measurement data signal that does not actually relate to the variable being measured. Thus, improved sensor designs have limited the information available for extraction from the measurement signal for fault detection purposes.
Recent use of digital communications technology by control system designers has enabled sensors to transmit multiple signals. This, in combination with internal diagnostics generated by microprocessors, which are now commonly embedded within sensors, has resulted in sensors that are able to perform fault detection analyses internally and transmit the results of these analyses as a fault information signal. Typically, the fault information signal is either a device specific error code or a single bit which indicates that the sensor is either functional or nonfunctional.